ARBUSCULAR MYCORRHIZAL PERCENT ROOT INFECTION AND INFECTION INTENSITYOF BROMUS HORDEACEUS GROWN IN ELEVATED ATMOSPHERIC CO2

Using Bromus hordeaceus, a grass from a Mediterranean annual grassland in California, we measured changes in infection intensity, rather tha n the more traditional % root infection, as an indicator of response t o elevated atmospheric CO2 and soil nutrient enrichment. Intensity was measured as the number of intraradical hyphae intersecting a microsco pe cross-hair for specific root diameter size classes. We found an inc rease in intensity of infection when plants were exposed to elevated C O2, and we found a decrease in infection intensity when plants were fe rtilized. This finding is significant in that it provides evidence for an increase in carbon allocation to the mycobiont under elevated CO2 even in the absence of change in percent infection, or mycorrhizal roo t length. Previous studies may therefore have overlooked an important response of arbuscular mycorrhizal fungi to this treatment, leading to an underestimation of the importance of mycorrhizae under elevated CO 2. Infection intensity may also change in response to many other treat ments and environmental variables that the symbiosis is exposed to, hi gh-lighting the potential usefulness of intensity as a response variab le in mycorrhizal research.